Re: [PATCH 0/6] Drain remote per-cpu directly v3

[Qian Cai]

On Thu, May 12, 2022 at 09:50:37AM +0100, Mel Gorman wrote:
> Changelog since v2
> o More conversions from page->lru to page->[pcp_list|buddy_list]
> o Additional test results in changelogs
> 
> Changelog since v1
> o Fix unsafe RT locking scheme
> o Use spin_trylock on UP PREEMPT_RT
> 
> This series has the same intent as Nicolas' series "mm/page_alloc: Remote
> per-cpu lists drain support" -- avoid interference of a high priority
> task due to a workqueue item draining per-cpu page lists. While many
> workloads can tolerate a brief interruption, it may be cause a real-time
> task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum,
> the draining in non-deterministic.
> 
> Currently an IRQ-safe local_lock protects the page allocator per-cpu lists.
> The local_lock on its own prevents migration and the IRQ disabling protects
> from corruption due to an interrupt arriving while a page allocation is
> in progress. The locking is inherently unsafe for remote access unless
> the CPU is hot-removed.
> 
> This series adjusts the locking. A spinlock is added to struct
> per_cpu_pages to protect the list contents while local_lock_irq continues
> to prevent migration and IRQ reentry. This allows a remote CPU to safely
> drain a remote per-cpu list.
> 
> This series is a partial series. Follow-on work should allow the
> local_irq_save to be converted to a local_irq to avoid IRQs being
> disabled/enabled in most cases. Consequently, there are some TODO comments
> highlighting the places that would change if local_irq was used. However,
> there are enough corner cases that it deserves a series on its own
> separated by one kernel release and the priority right now is to avoid
> interference of high priority tasks.

Reverting the whole series fixed an issue that offlining a memory
section blocking for hours on today's linux-next tree.

 __wait_rcu_gp
 synchronize_rcu at kernel/rcu/tree.c:3915
 lru_cache_disable at mm/swap.c:886
 __alloc_contig_migrate_range at mm/page_alloc.c:9078
 isolate_single_pageblock at mm/page_isolation.c:405
 start_isolate_page_range
 offline_pages
 memory_subsys_offline
 device_offline
 online_store
 dev_attr_store
 sysfs_kf_write
 kernfs_fop_write_iter
 new_sync_write
 vfs_write
 ksys_write
 __arm64_sys_write
 invoke_syscall
 el0_svc_common.constprop.0
 do_el0_svc
 el0_svc
 el0t_64_sync_handler
 el0t_64_sync

For full disclosure, I have also reverted the commit 0d523026abd4
("mm/page_alloc: fix tracepoint mm_page_alloc_zone_locked()"), so the
series can be reverted cleanly. But, I can't see how the commit
0d523026abd4 could cause this issue at all.